This invention relates to a motor control system for a motor driven pressure modulator and particularly to such a system wherein the torque of the motor is controlled to limit the speed of the motor.
The use of motor driven pressure modulators is known. For example, such uses include a motor driven braking pressure modulator in vehicle anti-lock braking systems. In these systems, during anti-lock controlled braking a DC torque motor drives a piston in a cylinder whose volume is modulated to control the hydraulic pressure at the wheel brake. In one such system, the motor is controlled to position the piston in an initial, fully extended home position at which a check valve is unseated to couple the brake system master cylinder to the wheel brake to allow normal braking. When anti-lock brake pressure modulation is required, the motor retracts the piston (which allows the check valve to close to isolate the master cylinder from the wheel brake as long as the wheel brake pressure is greater than the master cylinder pressure) to reduce brake pressure and thereafter modulates the piston position to provide pressure control for anti-lock braking. When anti-lock braking is no longer required, the motor returns the piston to its extended home position. While controlling the pressure, there is a direct relationship between the motor current, motor torque and the hydraulic pressure acting on the head of the piston.
It is desirable to control the speed of the motor in a motor driven pressure modulator such as described above. For example, it is desirable to limit the speed at which the motor repositions the piston to the extended home position such as upon termination of anti-lock controlled braking so as to prevent damage to the modulator when the piston head reaches its end of travel. However, an unknown speed dependent factor when rehoming the piston is the brake pressure on the head of the piston.
When rehoming the piston to its extended position at the termination of anti-lock braking, the pressure at the piston head is established by the vehicle operator via the normal brake system and may vary from no pressure to a relatively high braking pressure. Therefore, the speed of the motor for a given motor current command may vary widely. For example, when the motor load (i.e., the operator applied hydraulic brake pressure on the piston head) is equal to the motor torque, the motor does not rotate and the piston remains stationary. Conversely, when the load on the motor is small compared to the motor torque, the motor rotates at a high rate and the piston travels at a high speed. Therefore, care must be taken in the control of a motor driven pressure modulator to avoid situations in which the actual wheel brake pressure is substantially lower than the motor torque being commanded. When such a condition exists, motor torque must be controlled so as to prevent damage to the actuator when it reaches its end of travel.